The present invention generally relates to shading correction systems for image scanning apparatuses, and more particularly to a shading correction system for an image scanning apparatus such as a copying apparatus, a document reading apparatus including optical character recognition (ORC) and optical mark recognition (OMR) devices, a television camera and the like.
In the present specification, an image scanning apparatus refers to an apparatus which scans an image, and includes copying apparatus, a document reading apparatus including ORC and OMR devices, a television camera and the like.
Generally, in an image scanning apparatus which uses a line image sensor comprising photodetector elements arranged in a line (array), a light from a light source scans each of successive lines of an image such as a document along a main scanning direction, and the image information is read in groups of picture element data related to each line by detecting the reflected light from the scanned image in the line image sensor. However, due to the non-uniform light distribution characteristic of the light source and the inconsistency in the sensitivities of the photodetector elements constituting the line image sensor such as charge coupled devices (CCDs), an output signal level of the line image sensor changes and the so-called shading occurs. For this reason, it is necessary to carry out a shading correction so as to keep the output level of the line image sensor constant.
Conventionally, the shading correction is carried out as follows. That is, a white substrate is scanned before scanning the intended image which is to be scanned, and the output of the line image sensor is passed through an analog-to-digital (A/D) converter and stored into a random access memory (RAM) as shading data. Then, the intended image is scanned and the output of the line image sensor which is passed through the A/D converter is subjected to a predetermined operation by use of corresponding shading data read out from the RAM.
When the white substrate is scanned only once, the shading data which are obtained may contain errors caused by dust particles, paint flaw and the like on the white substrate at the scanned position. For this reason, it is desirable that the white substrate is scanned a plurality of times at different locations on the white substrate so as to obtain more accurate shading data. Hence, the shading data are conventionally obtained by scanning the white substrate a plurality of times and storing the shading data obtained for each scan, and constantly replacing a previous shading datum by a present shading datum when the present shading datum is greater than (more on the white level side of) the previous shading datum. As a result, maximum shading data obtained during the plurality of scans are read out from the RAM and used for carrying out the shading correction on the image information which is obtained from the line image sensor when the intended image is scanned.
The shading correction described above is superior compared to the case where the shading correction is carried out based on the shading data obtained during a single scan of the white substrate. Hence, even when the white substrate is partially dirty, dust particles or the like exist on the white substrate, the reflection coefficient of the white substrate is inconsistent and the like, the undesirable effects of such deficiency of the white substrate are minimized and it is possible to carry out an accurate shading correction.
When scanning the intended image, it is only necessary to read out the stored shading data from the RAM for the purpose of carrying out the shading correction based on the read out shading data. However, when scanning the white substrate and writing the shading data into the RAM, it is necessary to read out the previous shading data from the RAM, compare the previous shading data with the present shading data, and replace the previous shading datum by the present shading datum depending on the result of the comparison. In other words, the content of the RAM must be renewed at a speed in accordance with the scanning speed of the line image sensor, and there is a problem in that it is necessary to use a RAM which can operate at an extremely high speed. Furthermore, in order to make access to the RAM at the extremely high speed, the circuit construction of peripheral circuits of the RAM becomes complex. However, a RAM which operates at such an extremely high speed is expensive.